This invention relates to a high dynamic range interferometric spectrum analyser with direction finding capabilities for use in determining the position and direction of movement of an object such as an airplane.
Optical spectrum analysers have many applications, particularly in electronic warfare. Electronic warfare systems are required to measure angle of arrival of threat emitters over a wide frequency bandwidth and a pulse by pulse basis in the presence of simultaneous signals. In order to obtain accurate angle of arrival, phase interferometric systems must be used. The present invention is related to such application. There are some optical spectrum analysers with direction finding capabilities presently available. The most common ones are the Multichannel Bragg cells and the Mach-Zehnder interferometric spectrum analysers. The Multichannel Bragg cells produce direction of arrival data on a y-axis of a Fourier plane through the interference of the light diffracted by the channels. The position of a spot of diffracted light on the x-axis is associated with the frequency of the signal and the position of the light pattern on the y-axis is associated with the direction of arrival of the signal.
There are some drawbacks associated with this type of application. In order to properly generate the direction of arrival data, the relative phase of the laser illumination of the channels is required to be precisely controlled. This precision and of the RF signals applied to the channels is difficult to achieve. Also the spacing of the antenna and of the channels of a Multichannel Bragg cell are proportional and could lead to excessive cross-talk between too closely spaced channels.
Mach-Zehnder type interferometric spectrum analysers give a dynamic range by mixing the spectra of a detected signal with the spectra of a reference signal. Both signals are fed into Bragg cells located in the arms of an inteferometric structure in such a way as to produce a frequency offset between the two spectra. The direction of arrival is obtained by comparing the phase of the beat signals produced by the element of each detector array that is activated by the incoming signal. The number of antennas to be utilized depends on the wavelength of the received signal and on the ambiguity that one is willing to tolerate in the direction of arrival data. The frequency of the signal is determined by the particular position, along the x-axis, of the activated elements.
The main drawbacks of such systems is that large systems are difficult to align and to keep aligned. Furthermore, such systems are very susceptible to problems due to vibration because of the size and the number of components involved.
It is an object of the present invention to provide a high dynamic range spectrum analyser with direction finding capabilities that will be more compact and more resistant to vibrations than those currently available.